Inside a function, the second ; in a double ;; is a null statement, but
outside of functions a double ;; is simply invalid C that compilers
happen to accept. v4l2_m2m_enc.c contained several ;; as a result of
macro-expansion. So change the underlying macro so that it doesn't
happen any longer.

This fixes warnings when compiling with -pedantic: "ISO C does not allow
extra ‘;’ outside of a function".
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>